The invention relates to industrial furnaces providing heat treatment on a variety of articles, such as metal pieces, and in particular travelling strip metal. More precisely, the invention relates to leak testing water/gas heat exchangers used for cooling the atmosphere inside such furnaces in zones where such cooling is necessary.
As a general rule, steel strip is coated by being dipped in initially liquid zinc (galvanization) in an installation designed to galvanize travelling strip material on a continuous basis. Such an installation comprises firstly an annealing furnace in which the strip is successively heated and cooled. On leaving this furnace, the strip passes into a xe2x80x9cbellxe2x80x9d, which is a tubular piece connected to the furnace and having a bottom end through which the strip leaves to plunge into the bath of liquid zinc. The function of the bell is to isolate the travelling strip from the outside atmosphere between leaving the annealing furnace and entering the bath of zinc. The strip is then extracted vertically from the bath of zinc, at which stage it has a coating of zinc on both faces to a thickness which is subsequently equalized by passing between rollers.
Passing the strip through the annealing furnace is an essential step for obtaining high quality galvanization. It is essential to maintain a non-oxidizing atmosphere in the annealing furnace so that no scale forms on the surface of the steel strip. If any such scale were to form it would have disastrous consequences for the adhesion of the zinc deposit on the surface of the strip. To this end, the atmosphere throughout the inside of the furnace is constrained to comprise mostly nitrogen, while also containing about 2% hydrogen or even more for reacting with any traces of oxygen that might be present. That is why the gas constituting this atmosphere is conventionally referred to as xe2x80x9cHNxxe2x80x9d.
The annealing furnace is made up of a plurality of successive zones that are not sealed from one another and in which different temperatures are imposed, initially increasing and subsequently decreasing, and in particular:
a zone in which the strip is preheated;
a heating zone in which the strip reaches its maximum temperature;
a slow cooling zone; and
a fast cooling zone.
Furnaces for continuously annealing steel strip that are not located at the head of a galvanizing line can also operate on the same principle, terminating in one or more cooling zones.
In the preheating and heating zones, the desired temperature is obtained by injecting HNx that has been preheated to the desired temperature. This HNx then passes through the cooling zones where it must itself be cooled so as to perform its function. For this purpose, the cooling zones of the furnace include a succession of devices placed in register with each of the two faces of the strip for the purpose of extracting a fraction of the gas present in the zone of the furnace through which the strip is passing, then passing the gas through a heat exchanger and reinjecting the cooled gas back into the furnace, in the vicinity of the strip and downstream from the preceding extraction zone so as to enable the strip to be cooled over a fraction of its path. The heat exchangers which cool the extracted gas are conventionally constituted by copper coils having cold water circulating therein, with the gas to be cooled flowing around the coils. These coils are therefore subjected to large-amplitude thermocycling which damages them over time. In particular, regions of brazing constitute weak points where cracks are particularly likely to occur quite quickly. The consequence of such cracks is that the cooling water escapes from the coil and comes into contact with the gas to be cooled. It vaporizes and is injected back into the furnace together with the gas, thereby moistening the atmosphere in the furnace. When the water vapor content of the atmosphere becomes too high because of these leaks, the surface of the metal oxidizes, and as a result galvanization takes place poorly. Under such circumstances, it is necessary to replace the heat exchangers periodically. However, since such an operation requires the annealing furnace to be stopped for a long time (24 hours are needed to replace one heat exchanger), it must be performed only when absolutely necessary, since otherwise the productivity of the installation will be excessively compromised.
To determine when it is necessary to replace a heat exchanger, it is common practice to monitor the quality of the atmosphere in the furnace in each of its zones by periodically measuring its moisture content by determining its dew point. Typically, the dew point of a dry HNx atmosphere is xe2x88x9230xc2x0 C. to xe2x88x9240xc2x0 C. It is measured with a hygrometer, generally of the capacitive type. When a heat exchanger has a microcrack delivering about 0.1 liters per hour (1/h) of water into the atmosphere of the furnace, the dew point is lowered by about 0.6xc2x0 C. around xe2x88x9240xc2x0 C. Detecting such a small quantity of moisture is not within the ability of conventional hygrometers, even though such a small quantity is already sufficient to damage the surface of the strip. Ordinary measuring devices can be used only to detect the existence of relatively large leaks of water (about 0.2 l/h, which lowers the dew point of HNx by 1xc2x0 C. at around xe2x88x9235xc2x0 C.), even though such a leak can already have damaged a significant length of strip during earlier stages while the water content was building up. In addition, conventional devices do not make it possible to determine which heat exchanger is faulty. When a significant drop in the dew point is detected, it is therefore necessary to stop the annealing installation (and also the galvanization installation following it, if there is one) and to inspect the heat exchangers one by one, e.g. by increasing the pressure of the water circulating therein. That method is not suitable for detecting microcracks reliably, and in addition it accelerates damage to faulty brazing that has not yet become cracked through.
The object of the invention is to provide the users of furnaces that include water/gas heat exchangers for cooling the atmosphere thereof with a device for leak testing such heat exchangers that is suitable for detecting leaks at an early stage of development, while also identifying which heat exchanger is faulty.
To this end, the invention provides an installation for leak testing water/gas heat exchangers fitted to an industrial furnace, said furnace having at least one assembly formed by means for extracting hot gas present in a given zone of said furnace, a water/gas heat exchanger for cooling said hot gas, and means for reinjecting said cooled hot gas into a zone of said furnace situated downstream from said zone from which said hot gas was extracted, the installation being characterized in that it comprises:
means for sampling said hot gas at points each situated upstream from a heat exchanger and two-port valves allowing said sampling to be performed;
means for sampling said cooled hot gas at points each situated downstream from a heat exchanger and two-port valves allowing said sampling to be performed;
means for regulating the respective pressures of said hot gas and said cooled gas downstream from said sampling means;
means such as a three-port valve and its outlet pipe for selecting said hot gas or said cooled gas and applying it to a hygrometer; and
means for informing an operator of the results of analyses performed by the hygrometer and enabling the operator to compare the degree of humidity of said hot gas and of said cooled gas for each of the heat exchangers.
As will have been understood, the invention relies on the principle of differential measurement of the dew point upstream and downstream from each heat exchanger. Such a measurement requires a high sensitivity hygrometer to be used that is capable of detecting differences of about 0.3xc2x0 C. between dew points at around xe2x88x9235xc2x0 C. For this purpose, it is preferable to use an infrared absorption hygrometer.